Gaseous conduction method and apparatus



Oct. 3, 1933. Q $M|TH 1,929,123

GASEOUS CONDUCTION METHOD AND APPARATUS Original Filed May 24 1926 Patented Oct. 3, 1933 UNITED STATES GASEOUS CONDUCTION ZME'IHOD AND APPARATUS Charles G. Smith, Medford, Mass., was... to Raytheon Inc., Cambridge, Mass., a corporation of Massachusetts Application May 24, 1926, Serial No. 111,279 Renewed August 3, 1933 3 Claims. (01. 250-275) As is well known an arc discharge has characteristics distinguishing it from a glow discharge, namely, a voltage drop between cathode and anode below that required to maintain a 5 glow discharge, no cathode dark space, no socalled cathode voltage drop, etc. In order to produce a discharge having arc characteristics it has heretofore been deemed essential to employ an incandescent cathode or a mercury cathode having an incandescent spot, that is, a cathode heated to such high temperature that its thermionic emission is, by virtue of the high temperature, sufliciently great to produce an are discharge. Such high temperatures result in rapid deterioration and greatly reduce the period of normal operativeness.

According to this invention an arc discharge is produced independently of such high temperatures although the'cathode may in some cases incidentally become suiflciently hot materially to contribute to the arc characteristics. That is, this invention marks a radical improvement in the art in that it affords an arc discharge between non-thermionic electrodes, that is, electrodes which are not hot enough to maintain an are by the usual thermionic emission phenomena. Indeed the cathode may have such a low temperature as not to emit thermionically in substantial degree so that the arc is substantially independent of thermionic emission from the cathode. This unique result is attained by thermally ionizing the gas in the region of the active surface of the cathode and impeding the dissipation or escape of the ionization from said region sufficiently to cause the discharge between the non-thermionic electrodes to assume arc characteristics. By means of light or heat radiation or both this ionization is made sufliciently intense to produce ions and electrons capable of carrying the discharge current.

The invention will be best understood from the following description of the principles and exemplifications thereof, reference being made to the accompanying diagrammatic drawing wherein Figs. 1 and 2 are vertical sectional views illustrating two embodiments of the invention; Fig. 3 is a section on line 33 of Fig. 2; and Fig. 4 is a section on line 4-4 of Fig. 1.

If a perfectly reflecting and thermally nonconducting shield were available for confining the arc stream and if the electrodes could be made perfect in these respects, the drop in the are theoretically should be near zero. While the present invention does not attain this theoretical 5 state of perfection, it approaches it sufliciently to afford an are which is not dependent upon the usual thermionic emission, so that the cathode need not have a hot spot but may remain comparatively cold.

In accordance with the invention, ionization in the region of the active cathode surface may be effected by radiation from an incandescent filament or rod which is maintained at incandescence by an auxiliary current independent of the cathode-anode discharge. -A hollow cathode with a restricted discharge opening is preferably used and the ionization is maintained in a gas therein by thermal agitation of the gas, accompanied by radiation from the cathode-anode discharge, owing to the small loss through the re- .70 stricted opening. When no auxiliary radiation means is employed a higher potential is required to start the discharge, namely, a potential high enough to produce a glow discharge, the voltage dropping as soon as the ionization becomes sufll- 76 ciently intense to give the discharge are characteristics.

While the phenomena by which an arc may be maintained between non-incandescent electrodes according to the present invention are 80 not fully understood tests have conclusively demonstrated that it is not dependent upon thermionic emission from the cathode. For example,

I have drawn an arc discharge from the interior of a hollow molybdenum lined cathode to a plane anode, in mercury vapor at about one centimeter pressure inside the cathode, with the cathode no hotter than dull red and with only a few volts drop between cathode and anode. In this case there was no localized hot-spot as in the case of prior art arcs. Under these circumstances it is inconceivable that the arc was influenced substantially by thermionic emission.

In practice the cathode is usually not in strict thermal radiative equilibrium with the surrounding gas, and energy must be supplied to make up the energy losses at the cathode. Energy is also lost by radiation from the boundary of the arc inasmuch as it is, of course, impossible to conserve all the radiation by reflection or otherwise. Consequently a certain voltage drop between cathode and anode, while markedly low, must exist in greater or lesser degree depending upon the degree of efficiency with which the radiation is conserved. In the aforesaid example of mercury vapor inclosed by reflecting molybdenum, it is probable that much of the radiation of longer wave length than 2000 Angstrom units is conserved. One form of arrangement according to the invention is shown in Figs. 1 and 4.

The arrangement comprises a discharge tube in the form of envelope 10 enclosing a hollow cylindrical cathode 23 supported from the upper closed end of the tube. 'The cathode 23 has its open end 24 disposed close to the upper end of a conduit 25, the lower end of which dips into the vaporizable liquid 14 such as mercury, caesium, or other alkali metal, or other suitable substance. The filamentary heating element 26 is connected by supporting leads 2'! and 28 to winding 29 of transformer 30 which is supplied with power from the source of alternating current 17. Filament 26 is preferably constructed to operate at a high temperature, such as at a bluish white heat, with a relatively low voltage applied thereto, thus preventing on of space current between the filament terminals by the voltage drop thereim The filament is wound to have a spiral concave shape which causes intense radiation to emanate into the cathode 23 and impinge upon its inner surface, the filament being spaced close to the opening 24 in the chamber in order to transmit its radiations most efiiciently into the cathode. The heat from filament 26 causes also vapor to rise within conduit 25 and discharge into chamber 23 wherein the intensified radiation from the filament ionizes the vapor without requiring an excessively high temperature in the a material of the chamber. In this construction it is possible to utilize a material for'cathode 23 which may be photoelectrically excited by radiation from filament 26, the emission from the cathode supplementing the emission from the vapor. Since the cathode is not raised to as high a temperature as that corresponding to the radiation exciting the photoelectric eiIect and the ionization of the vapor, a material of relatively low melting point, such as calcium, for example, or other substance, may be utilized for the cathode instead of molybdenum or similar metal, referred to before.

By using the filament 26, space current is readily established between the cathode 23 and the mode 31 positioned close to the cathode adjacent the open end 24. When mercury or other material requiring high temperatures for ionization are employed in the tube, the variable resistance 32 is connected between the filament and cathode 23. This resistance is preferably adjusted to a high value to prevent fiow of appreciable space current to filament 26.

By positioning filament 26 in line with the opening between cathode 23 and conduit 25 through which space current passes to anode 31,

the-space current path acUacent the anode is intensely illuminated. Negative ions in the path of the space current adjacent the anode, tending to increase the resistance of the space current path at the anode, are destroyed by radiation from filament 28 thus reducing the drop of potential of the anode, and permitting the rectifier to operate efiiciently at a still lower voltage.

The embodiment shown in Figs. 2 and 3 comprises a cup-shaped metallic tube having a cover 41, which may be of the same material,

and spaced from the plugs 44 distances substantially confined to the mean free path of electrons in the gas, thereby to protect the insulation from disintegration during operation of the device. The cathode is in the form of a cylindrical box, of nickel, iron, or the like, preferably with a fiaring mouth 55 of restricted diameter; and the anodes are in the form of plates as shown. Resting on the bottom of the tube 40 and held in position in any suitable way is a cylinder 48 having a restricted mouth 49 at its upper end and perforations 50 near its lower end, this cylinder being surrounded by a cylinder 51 of lava or other refractory insulation having openings registering with perforations 50. Within a reentrant portion 52 of tube 40 is a heating coil 53. The lower end or the tube is filled, preferably to a level somewhat above the portion 52, with material 54 such as metallic caesium which when heated gives off a readily ionizable'vapor. After the tube is evacuated an inert gas may be admitted at a low pressure but for most purposes the vapor of casium or the like may be used alone.

The operation of the tube shown in Figs. 2 and 4 is as follows: By means of the cathode-anode discharge, supplemented by the auxiliary heater 53, the body of ciesium 5.4 is melted and gives ofi' caesium vapor which is readily ionizable. The heat within the cylinder 48 is sufiiciently intense to produce sufiicient pressure to cause the vapor to issue from the mouth 49 in the form of a jet which passes into the hollow cathode 42 through the mouth 55 and produces a pressure within the cathode higher than outside the cathode. This results in a higher degree of ionization within the cathode and owing to the fact that the cathode has only a small opening the radiation within the cathode is conserved. This conservation may be enhanced by making the interior of the cathode reflecting; the radiation from caesium being readily reflected since itihas only relatively long wave lengths. The gaseous discharge passes between the interior of the cathode and the anodes through the opening 55.

and the discharge assumes arc characteristics even though the cathode does not become hot enough to maintain an are by thermionic emission alone. This is believed to be due largely to the fact that thetotal intensity of ionization in- -side the cathode is so great that the number ions rebounding therefrom with one or more electrons combined therewith. The electrons combined with positive ions at or near the cathode are released sooner or later by the aforesaid ionizing agencies. Other electrons are pulled bodily from the cathode and pass out into the gas. Still other electrons are liberated photoelectrically from the cathode. Irrespective of which of theseactions is-predominant, the dark space adjacent the cathode is eliminated and the voltage drop is that of an arc in contradistinction to a glow discharge, notwithstanding the cathode is below the incandescent temperature heretofore thought necessary for an arc discharge.

The formation and maintenance of an are from a relatively cold cathode according to the Electrons are liberated at the cathode by the great number of positive present invention is believed to depend largely upon radiation in the region of the active cathode surface. In this connection it should be borne in mind that the recombination of ions and electrons produces radiation and that radiation of high frequency will produce ions. Consequently, after a high'degree of ionization is established the recombination of ions and electrons produces radiation which, if conserved in the region of the active surface of cathode, is sufficient to maintain the arc with the low voltage drop characteristic of the present invention. It seems probable that the ionization is so intense that the gas is practically in equilibrum with its radiation. While the usual photometric or spectroscopic observation does not indicate such a high degree of ionization, this is undoubtedly due to the fact that the outer cooler layers of the gas obstruct accurate measurement of the central region of the ionized mass of gas.

A preferred way of starting an are such as herein described consists in first establishing a glow discharge by impressing a relatively high potential difference between the electrodes, thereby producing the initial ionization. By confining the discharge in sufficient degree according to the present invention, the thermal ionization in the gas, assisted by the accompanying radiation which is also confined, soon reaches a degree of intensity which suddenly causes the discharge to assume are characteristics as indicated by disappearance of the cathode dark space and an abrupt decrease in the voltage drop. This transition from glow discharge to are discharge is undoubtedly due to the presence of an enormous number of ions in the region of the active surface of the cathode, the initial production and the maintenance of such a high degree of ionization being due to the markedly efficient conservation of energy in the region of the discharge.

The production of an are by thermal ionization in the gas (rather than by thermionic emission from the cathode) is facilitated by employing a gas in which intense thermal ionization takes place at a relatively low temperature, that is by using a gas having a low ionization voltage. In this respect caesium vapor is preeminent inasmuch as it has an extremely low ionization voltage. Another factor in producing an are predominantly by thermal ionization involves the wave length of the radiation incident to the thermal ionization. The longer the wave length the more easily the radiation can be reflected and conserved. Consequently by using a gas which affords relatively long wave radiation the conservation of radiation by reflection contributes much more effectively to the intense ionization by which the arc is maintained according to the present invention. Caesium is also superior in this respect since its radiation has unusually long wave length.

The phenomena attending the action of this unique arc involves such abstruse factors as equilibrium relations in a medium filled with neutral molecules, electrons, radiations of various wave lengths, atoms in various stages of preparedness, and the interdependence of these factors; and there is no known way of accurately and completely determining the theory of operation of the arc. However, from many observations of arcs produced according to the present invention I believe this unique arc (is maintained partly by the ions in the intensely ionized gas near the cathode rebounding from the cathode in their molecular movement in the region of the oathode and carrying away electrons, the resul ing neutral molecules being again ionized in the space between the electrodes, and the electrons ultimately being attracted to the anode. Furthermore, some electrons are undoubtedly pulled out of the cathode by the dense aggregate of ions near the cathode surface; and other electrons are unquestionably liberated photoelectrically by the intense radiation. The aforesaid phenomena may or may not be attended by thermionic emission of electrons in substantial degree depending upon the temperature of the cathode, the pro.- duction of an arc according to the present invention not being dependent upon thermionic emission.

In the case of prior low voltage arcs of the hot-spot type the ionization energy close to the cathode surface is supplied by energy from the cathode in the form of radiation, conduction, electron evaporation etc whereas in the case of an are from a relatively cold cathode according to the present invention the ionization is produced predominantly by thermal agitation and radiation in the gas or vapor.

I do not wish to be limited to any particular theory underlying the operation of my improved discharge device described above, nor to any details of construction, as many equivalents may suggest themselves to those slnilled in the art. I accordingly desire that the appended claims be given a broad construction commensurate with the scope of the invention in the art.

I claim:

1. An electrical high-current space discharge device comprising a gas tight vessel enclosing an anode, a hollow cathode structure having an extended interior solid electrode surface, an ionizable gas sufficient in amount to have during operation a pressure high enough to sustain between said anode and cathode a discharge at a voltage of the order of the ionizing voltage of the gas or less, said cathode structure constituting an enclosure around the discharge space in front of said cathode surface and having a discharge opening to the anode and screening during operation the enclosed gas adjacent to said cathode surface against loss of heat and ionization, and means for maintaining during operation the gas in said enclosure at a temperature and ionization sufiiciently high to secure a discharge between the anode and cathode at a voltage of the order of the ionization voltage of the gas or less,

the enclosure formed by said cathode structure being sufficiently opaque and covering to a sufficient degree the space adjacent said cathode surface to secure conduction at said low voltages while maintaining the electrode surface of said cathode below temperature of thermionic emis- 2. An electrical high-current space discharge device comprising a gas tight vessel enclosing an anode, a hollow cathode structure having an extended interior solid electrode surface, said cathode structure constituting an enclosure around the discharge space in front of said cathode surface and having a discharge opening to the anode and screening the space in front of the cathode surface against loss of heat and ionization, a vaporizable substance for supplying an ionizable vapor to said enclosure positioned to prevent thereon formation of a cathode spot for the discharge, means including a heating element mounted to be independently heated to incand'escence for supplying during operation vapor from said substance to the interior of said cathode enclosure and for maintaining said vapor in said enclosure ionized at sufficiently high temperature and enough pressure materially higher than in the anode region to produce a discharge between said cathode structure and said anode age of the vapor orless, theencloeure-formed by said cathode structure being eufll'cientlyopaque and covering-the cathode surface to-a suflicient degree to secure the discharge at low voltage while said cathode surface is at'a temperature below thermionic emission.

'3'. An electrical high-current space discharge device comprising a gas-tight vessel enclosing an solid electrode surface, ionizablegassumcientin enough to sustain between said anode and catheague odeadischargeatavoltageoltheorderotthe ionising voltage ot'thegas or less. the discharge space in iront'oi' said cathode surface being screened during operation against the loss of heat'and ionization, and meansfor maintaining during operation the gas adjacent said cathode I surtaoe'at a temperature and ionization mmciently high to secure a discharge between the anode and cathode at a voltage of the order of anode, a cathode structure having an extended"- the ionization voltage ofthegas or less while maintaining the electrode surface of said cathode amount'to have during operation I pressure-highthermionic emission.

below temperature of CHARLES G. SMITH. 

